The Invention relates to a method for detecting an information signal, tone and/or a phase change of a tone in one or more signals which contain inter alia this information signal or this tone.
Inband signalling with special signals, primarily sinusoidal tones, is used in many areas of telecommunication, for example in switching systems. A specific frequency must be reliably detected from a mixture of sinusoidal frequencies which are superimposed by noise or other signals and it must optionally also be detected whether the sinusoidal oscillation contains a phase change or not.
It is known to detect a tone by band filters or by analysing the signal in the frequency range by computing the discrete Fourier transforms of the signal or a signal block. The principle of the known algorithms for detecting a phase change consists in comparing the instantaneous values of the incoming tone with a version which has been time delayed by an exact multiple of the period length, as is known from EP 0 565 672 B1 or from the article “A simple phase-reversal tone disabler” by J. L. Melsa and R. J. Ragland in the conference proceedings from the ICASSP 86 Conference, Tokyo, pages 2619 to 2622. Owing to frequency tolerances and strong noise interference, a critical point for the algorithm lies in the estimation of the correct period length. The number of computation operations required is very high, so the known algorithms are not suitable owing to the high expenditure on hardware in computing units or digital signal processes (DSP), in order to process, for example, 2,000 channels in parallel for the purpose of detecting tones and/or phase changes.